High-concentration solar cell chip

ABSTRACT

A high-concentration solar cell includes an epitaxial layer structure, an upper patterned electrode on the top surface, and a back electrode on the back surface. The upper patterned electrode includes a primary pattern and a secondary pattern, where the primary pattern is composed of a series of small metal isosceles trapezoids around the perimeter of the cell. The narrower base of each metal trapezoid points toward an interior of the cell. A lead soldering pad is located within each metal trapezoid for being soldered to an external conductor for carrying the solar cell current. The secondary pattern consists of thin spaced conductors that connect to the angled sides and base of each trapezoid and spread current across the top surface of the cell. The current along the angled sides of each trapezoid is well-distributed to all the spaced conductors connected to the angled sides to avoid current crowding.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims priority to PCT Application CN2012/075135, filedon May 7, 2012 and published on Jan. 3, 2013 as publication WO2013000339A1, which claims priority to Chinese Patent Application No.201110176319.6 titled “A High-Concentration Solar Cell Chip”, and filedwith the Chinese Patent Office on Jun. 28, 2011, which is herebyincorporated by reference in its entirety.

FIELD OF THE INVENTION

The present invention relates to a high-concentration solar cell chip.

BACKGROUND OF THE INVENTION

Presently, solar cells for concentrated photovoltaics normally use agallium arsenide (GaAs)-based multi-junction solar cell. Itsconcentration factor can be as large as 500×, or even in the scale ofthousands, due to the advancement of material performance and processingtechnology. Moreover, its chip size is much smaller than traditionalsilicon-based cells, greatly reducing the semiconductor materialconsumption and making it the most promising solar cell. However, thephotocurrent generated by solar cell chip will be in direct proportionto the concentration ratios. For example, by a thousand of concentrationfactors, three-junction concentrated solar cell will produce ahigh-density current at 15-20 A/cm2, which requires that the seriesresistance should be small enough, and also the current distributionshould be well-distributed, in order to decrease the resistive loss ofthe cell and avoid the local overheating caused by current crowdingwhich seriously affects the cell's reliability. If the sunlightdistributes uniformly on chip's surface, the current will also be evenlydistributed in the chip epitaxial structure and back electrode, but thecurrent crowding problems with the upper electrode, especially theprimary grid, would not be changed due to the sunlight distribution.

As for the upper-electrode in the traditional grid, the primary grid hasan elongated and regular rectangular structure and a secondary gridevenly connects with the long edge of the rectangle. Considering thecurrent flowing from the secondary grid to the primary grid, it mustflow to the lead soldering region through the primary grid. From theview point of resistance, there are two extreme paths for the currentflowing through the primary grid: 1) after an outflow from the secondarygrid, directly flowing to the lead soldering region along one side ofthe primary grid closing to the secondary grid; and 2) after an outflowfrom the secondary grid, vertically flowing to the other side of primarygrid which is farther away from secondary grid, and then, to the leadsoldering region. Obviously, the current flows along the first shorterpath. Thus, all currents flowing from the secondary grid will flow alongone side of the primary grid closer to the secondary grid, while thedensity of current flowing along the other side which is farther awayfrom secondary grid is smaller. The uneven distribution of currents willcause current crowding, and under high-concentrated conditions, thiseffect will be more severe, which will lead to greater localoverheating.

FIG. 1 to FIG. 3 are schematic diagrams of the traditional electrodepattern and its current route. A and B (FIG. 2) are respectively themost distant and shortest extreme paths for the current from thesecondary grid to flow through the primary grid to the lead solderingregion. Obviously, the current tends to flow along the shorter path B.All currents flowing from the secondary grid will flow along one side ofthe primary grid closer to the secondary grid, which results in thecurrent crowding under high-density current conditions; in comparison,the other side of the primary grid which is farther away from thesecondary grid will have a smaller current density of, and thus willmake a smaller contribution to the photocurrent transmission and inessence sacrifice the effective illumination area on the cell's surface.

SUMMARY OF THE INVENTION

In order to solve the problem above, the present invention provides ahigh-concentration solar cell chip, the structure of which includes: anepitaxial layer structure, an upper patterned electrode on the uppersurface of the epitaxial layer structure, and a back electrode on thelower surface of the epitaxial layer structure. The upper patternedelectrode contains the primary grid and the secondary grid. The primarygrid is composed of a series of isosceles trapezoid structures.Trapezoidal upper-bases are in the same line pointing to the interior ofcell chip. The region from the upper-base of trapezoids to thelower-base is the lead soldering region. The secondary grid connects thetwo sides or the upper-bases of the isosceles trapezoids of the primarygrid.

Preferably, the number of isosceles trapezoids of the primary grid isequal to that of soldering wires on the primary grid. The region fromthe upper-base of the trapezoids to the lower-base is the lead solderingregion. The length of the upper-base is equal to the width of the leadsoldering region, which is perpendicular to the secondary grid. Thelength of the lower-base is equal to the spacing between the adjacentlead soldering regions.

Preferably, the spacing between the secondary grids is equal, and thesecondary grids evenly connect the two sides or the upper-base ofisosceles trapezoid of the primary grid.

Preferably, the isosceles trapezoids of the primary grid are arrangedsequentially, with the trapezoidal lower-bases on the same line, and allthe isosceles trapezoidal upper-bases, which point to the interior ofthe cell, on the same line.

Preferably, the primary grid is divided into two columns located on twoopposing sides of the solar cell, and the upper-bases of the saidisosceles trapezoids point to the interior of the cell.

Preferably, the primary grid is divided into four columns located on allfour sides of the solar cell, and the upper-bases of the isoscelestrapezoids point to the interior of the cell.

Preferably, as for the isosceles trapezoids of the primary grid, thelength of its upper-base is 0.1˜2 mm, the length of its lower-base is2˜5 mm, and the height of the isosceles trapezoids is 0.1˜1 mm.

Preferably, the upper patterned electrode includes: an ohmic contactlayer covering the epitaxial layer structure; an adhesion layer coveringthe ohmic contact layer; a conductive layer covering the adhesion layer;and a protective layer covering the conductive layer.

Preferably, the upper patterned electrode and the bare surface ofepitaxial layer are both covered with an anti-reflective coating.

Compared with the prior art, the present invention has advantages asfollows:

First, the upper patterned electrode as designed can effectively preventthe problem of current crowding. Due to the connection points betweenthe secondary grid and the primary grid being evenly distributed to twosides of the upper-base of the isosceles trapezoid, namely evenlydistributed along the vertical direction of primary grid, and theprinciple that current flow follows the shortest path, the currentflowing from the secondary grid to the primary grid will respectivelyflow along the shortest line to the lead soldering region, so as torealize the well-distributed spreading of current and avoid the problemof current crowding.

Second, the invention adds more effective illumination area on thecell's surface without the increase of resistive loss and currentcrowding, and thereby, improves the photocurrent size and thephotoelectric conversion efficiency.

Furthermore, the invention also includes an optimized design for theupper patterned electrode with the four-layer structure in order toensure good ohmic contact to the solar cell epitaxial layer and alsoensure good conductivity of the electrode. The adhesion layer promotesadhesion between the conductive layer and the ohmic contact layer andprevents any inter-diffusion between them from. The protective layerprotects the conductive layer from oxidation and contamination, and inaddition, it allows for wider selection of suitable materials forsoldering on the electric lead.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention is further described in details and not limitedhereafter in conjunction with the accompanying drawings and thepreferred embodiments. In addition, the drawings are descriptive and notdrawn to scale.

FIG. 1 is a plan view of the upper-electrode in the traditional grid;

FIG. 2 is a partial enlarged drawing of the upper-electrode in the gridshown in FIG. 1;

FIG. 3 is an electric flow path diagram of the traditional upperpatterned electrode shown in FIG. 1;

FIG. 4 is a cross-sectional view of a high-concentrated solar cell chipaccording to a preferred embodiment of the invention;

FIG. 5 is a plan view of the upper patterned electrode according to thefirst preferred embodiment of the invention;

FIG. 6 is a plan view of the upper patterned electrode according toanother preferred embodiment of the invention;

FIG. 7 is a partial enlarged drawing of the structure of primary grid inupper patterned electrode shown in FIG. 5;

FIG. 8 is an electric flow path diagram of the upper patterned electrodeshown in FIG. 5;

FIG. 9 is a side sectional view of the upper patterned electrodeaccording to a preferred embodiment of the invention.

Elements that are the same or similar are labeled with the same numeral.

Components in the drawings include:

100: Antireflection film

200: Upper patterned electrode

201: Primary grid

202: Lead soldering region

203: Secondary grid

204: Ohmic contact layer

205: Adhesion layer

206: Conductive layer

207: Protective Layer

300: Epitaxial layer structure

400: Back electrode

500: Electric flow path

DETAILED DESCRIPTION

An embodiment of present invention is further described in detailhereafter in conjunction with the accompanying drawings. The embodimentof the present invention and each feature of embodiment undernon-conflicting situations can be combined with each other, which iswithin the protection scope of the invention.

As shown in FIG. 4, a high-concentration solar cell chip includes anepitaxial layer structure 300 specifically designed for ahigh-concentration solar cell, which is of a III-V compound and can be aeither single-junction or multi-junction structure; an upper patternedelectrode 200 formed on the upper surface of the epitaxial layerstructure 300; and a back electrode 400 formed on the lower surface ofepitaxial layer structure 300. In a preferred embodiment of theinvention, the upper patterned electrode 200 and the exposed surface ofepitaxial layer 300 are both covered with an antireflection film 100.

As shown in FIG. 5 to FIG. 7, the upper patterned electrode 200comprises the primary grid 201 and the secondary grid 203; the primarygrid 201 is composed of a series of isosceles trapezoid structures, andtrapezoidal upper-bases b (FIG. 7) are in the same line and pointing tothe interior of cell. The region between the upper-bases of thetrapezoids and the lower-bases is the lead soldering region 202 forsoldering to an external conductor to carry the solar cell current. Thesecondary grid 203 has even spacing and connects either the two angledsides of the isosceles trapezoids or the upper-bases of the isoscelestrapezoids of the primary grid 201 to the sides and upper bases of othertrapezoid in the primary grid 201. According to a preferred embodimentof the invention, the number of isosceles trapezoids of the primary grid201 is equal to that of soldering wires on the primary grid; the regionbetween the upper-bases of the trapezoids and the lower-bases is thelead soldering region; the length L_(a) (FIG. 7) of the lower-base a(FIG. 7) is equal to the distance L between the adjacent lead solderingregions. The length L_(b) of the upper-base b of the trapezoids isdecided in terms of the size of the lead soldering region. According toa preferred embodiment of the invention, the length L_(b) of theupper-base b is equal to the height of the lead soldering regionperpendicular to the secondary grid. The length L_(b) of the upper-baseb of the isosceles trapezoid of the primary grid 201 is 0.1˜2 mm, thelength L_(a) of the lower-base a is 2˜5 mm, and the height h of theisosceles trapezoids is 0.1˜1 mm.

According to a preferred embodiment of the invention, the length of theupper-base of isosceles trapezoid is 0.5 mm, the length of thelower-base is 2.5 mm, and the height is 0.3 mm.

The primary grid 201 is constructed by a series of trapezoids, which canbe configured on either two sides or four sides of the cell. As shown inFIG. 5, when it is constructed in the two-side configuration, two rowsof trapezoids are distributed on two sides of the solar cell, andthereof, the upper-base b of isosceles trapezoids points to the interiorof the cell. As shown in FIG. 6, when it is constructed in the four-sideconfiguration, four rows of trapezoids are distributed on all four sidesof the solar cell, and the upper-base of isosceles trapezoids points tothe interior of the cell.

Due to the high current density in the solar cell underhigh-concentration conditions, there are stringent challenges on theohmic contact between the electrode and the epitaxial layer, theelectric conductivity of electrode, and the secondary electrode design.Therefore, the preferred embodiment of invention has adopted amultilayer electrode structure since a single metal layer cannot meetthe application requirements.

As shown in FIG. 9, the upper patterned electrode 200 comprises an ohmiccontact layer 204, an adhesion layer 205, a conductive layer 206, and aprotective layer 207.

The ohmic contact layer 204 covering the epitaxial layer structure 300is used to form a good ohmic contact with the epitaxial layer structure300 of the high-concentration solar cell. With a thickness of 10˜300 nm,it is made of Au Ge alloy, or germanium, or palladium, or anycombination thereof. According to a preferred embodiment of theinvention, the ohmic contact layer 204 with a thickness of 200 nm ismade of Au Ge alloy.

The adhesion layer 205 covering the ohmic contact layer 204 is used topromote adhesion between the ohmic contact layer 204 and conductivelayer 206. With a thickness of 1˜20 nm, it is made of titanium, ornickel or any combination thereof. According to a preferred embodimentof the invention, the adhesion layer 205 with a thickness of 10 nm ismade of titanium.

The conductive layer 206 covering the adhesion layer 205 with athickness of 1˜10 microns is made of high conductivity materials, suchas silver, aluminum. According to a preferred embodiment of theinvention, the conductive layer 206 with a thickness of 6 nm is made ofsilver.

The protective layer 207 covering the conductive layer 206 with athickness of 10˜200 nm is used to protect the conductive layer andprevent oxidation and contamination. Suitable materials for theprotective layer can be selected while taking into account therequirement for soldering of the electric leads. According to apreferred embodiment of the invention, the protective layer with athickness of 20 nm is made of gold.

All four layers 204˜207 of the upper patterned electrode 200 follow thesame pattern of upper patterned electrode 200.

FIG. 8 is an electric flow path diagram of the upper patterned electrodeaccording to a preferred embodiment of the invention. Due to theconnection points between the secondary grid 203 and the primary grid201 being evenly distributed on the two sides or the upper-base ofisosceles trapezoid, namely evenly along the vertical direction ofprimary grid, since electric current 500 will flow according to theshortest path principle, the current flowing from the secondary grid 203to the primary grid 201 will respectively flow along the shortest pathto the lead soldering region 202, so as to realize the well-distributedspreading of current 500 and avoid the problem of current crowding.

While particular embodiments of the present invention have been shownand described, it will be obvious to those skilled in the art thatchanges and modifications may be made without departing from thisinvention in its broader aspects and, therefore, the appended claims areto encompass within their scope all such changes and modifications thatare within the true spirit and scope of this invention.

What is claimed is:
 1. A solar cell, comprising: an epitaxial layerstructure; an upper patterned electrode on a top surface of theepitaxial layer structure; a back electrode on a back surface of theepitaxial layer structure, wherein a current is generated between theupper patterned electrode and the back electrode when the solar cellgenerates current;: the upper patterned electrode comprising a primaryelectrode pattern and a secondary electrode pattern, wherein the primaryelectrode pattern comprises a series of isosceles trapezoidalstructures, each isosceles trapezoidal structure having a lower basepointing away from the cell, having a narrower upper base pointingtoward an interior of the cell, and having angled sides between edges ofthe lower base and the upper base; wherein the upper bases of thetrapezoidal structures along a same edge of the cell are in a same line;a lead soldering region within each trapezoidal structure, each leadsoldering region being configured for being electrically connected to anexternal conductor to conduct current; and wherein the secondaryelectrode pattern comprises electrical connector lines extending overthe top surface of the epitaxial layer structure, some of the connectorlines extending from the angled sides of the isosceles trapezoidstructures and other ones of the connector lines extending from theupper bases of the isosceles trapezoid structures to spread current overthe top surface of the epitaxial layer structure, whereby, when thesolar cell is generating current, current flows between each leadsoldering region and the angled sides and upper base of its associatedtrapezoidal structure in the primary electrode pattern, and is spreadover the top surface of the epitaxial layer structure by the secondaryelectrode pattern.
 2. The solar cell according to claim 1, wherein alength of each upper base is substantially equal to a width of the leadsoldering region, and wherein a length of each lower base issubstantially equal to a spacing between adjacent lead solderingregions.
 3. The solar cell according to claim 1, wherein the connectorlines in the secondary electrode pattern have substantially uniformspacing.
 4. The solar cell according to claim 1, wherein the isoscelestrapezoidal structures of the primary electrode pattern are formedaround a perimeter of the top surface of the epitaxial layer structure.5. The solar cell according to claim 1, wherein the isoscelestrapezoidal structures of the primary electrode pattern are formed alongopposite edges of the top surface of the epitaxial layer structure. 6.The solar cell according to claim 1, wherein a length of each upper baseis 0.1˜2 mm, a length of each lower-base is 2˜5 mm, and a distancebetween the upper base and lower base of each isosceles trapezoidstructure is 0.1˜1 mm.
 7. The solar cell according to claim 1, wherein:the upper patterned electrode comprises: an ohmic contact layer coveringthe epitaxial layer structure; an adhesion layer covering the ohmiccontact layer; a conductive layer covering the adhesion layer; and aprotective layer covering the conductive layer.
 8. The solar cellaccording to claim 8, wherein the ohmic contact layer has a thickness of10˜300 nm, and comprises at least one of an AuGe alloy, germanium,palladium, or combinations thereof.
 9. The solar cell according to claim8, wherein the adhesion layer has a thickness of 1˜20 nm, and comprisesone of titanium or nickel.
 10. The solar cell according to claim 8,wherein the conductive layer has a thickness of 1˜10 microns, andcomprises a high-conductivity materials.
 11. The solar cell according toclaim 8, wherein the protective layer has a thickness of 10˜200 nm, andcomprises gold.
 12. The solar cell according to claim 1, wherein theupper patterned electrode and an exposed surface of the epitaxial layerstructure are covered with an antireflective film.
 13. The solar cellaccording to claim 1, wherein each trapezoidal structure comprises ametal layer having the angled sides and upper base, and the leadsoldering region for each trapezoidal structure is formed over the metallayer.
 14. The solar cell according to claim 13, wherein current thougheach lead soldering region is coupled to the angled sides and upper baseof its associated trapezoidal structure to a plurality of the connectorlines.
 15. The solar cell according to claim 14 wherein the angled sidesspread current to a plurality of the connector lines.